Skip to main content

Controlling combined sewer overflows

Controlling combined sewer overflows

King County and the City of Seattle working together

Since 1979, King County has reduced its combined sewer overflows (CSOs) by 90 percent. This has succeeded in keeping more than 2.3 billion gallons of sewage and stormwater out of local waterways.

King County is required to control all of its CSOs by 2030. A “controlled” CSO can overflow no more than one time each year, on long-term average, under the Washington State standard.

Stormwater Grate

King County and Seattle: both working to fix the problem

King County and the City of Seattle are each responsible for specific CSO relief points within the city limits. The history of the sewer system led to this shared responsibility. In the 1950s, voters created Metro to clean up the region’s waterways. Metro built a regional sewer system and took over operation of some of the City of Seattle's system. In 1994, King County assumed authority of Metro. King County is now responsible for treating wastewater for 34 local jurisdictions and agencies, including the City of Seattle. 

Each CSO provides a drainage relief point for specific neighborhoods. King County manages the CSOs that serve areas that are greater than 1000 acres. The City of Seattle manages the CSOs that serve smaller areas. King County manages 38 CSOs and Seattle manages about 85. View this map for more information.

Decades of successful improvements

King County and Seattle have reduced the amount of untreated wastewater released to waterways. In the 1960s, around 30 billion gallons of untreated water flowed into our waterways each year. Now King County releases less than one billion gallons. King County began its CSO Control Program (now called Protecting Our Waters) in 1979. From 1979 to 2012, the County invested $389 million. The amount of polluted water released each year went from 2.3 billion gallons to about 0.8 billion gallons. More projects are being planned and built to reduce this further. The remaining projects are some of the most complex and expensive. 

The following graph shows CSO reductions from the many control projects King County has completed:

A graph displaying estimated annual discharge of combined sewer overflow from King County's system over time. The "Y" axis is Billions of Gallons (0 to 30) and the "X" axis is time (1960 - 2030). When Metro became an operating wastewater agency (1960), the estimated flow was between 20-30 billion gallons. After numerous infrastructure projects, the estimated flow reduced to 600-900 million gallons (Year 2000) and is estimated to be between 150-300 million gallons in 2030, which corresponds to the WA standard of 1 untreated event/year/outall. Graph also available as PDF .

Janice Johnson
CSO Control Program